IC 8835 



Bureau of Mines Information Circular/1980 



Guide to Substation Grounding 

and Bonding for Mine Power Systems 

By Wils L. Cooley and Roger L. King 



UNITED STATES DEPARTMENT OF THE INTERIOR 



iju( u Iqm pctijoa^^ &J //1t~ 



"■■ ■ ■ i 



&J> / /lnii4 . ^ /7 y\ 



c?)/7tjce}y> : i4.t t -4>^ ^^ L ; 



Information Circular 8835 



Guide to Substation Grounding 

and Bonding for Mine Power Systems 

By Wils L. Cooley and Roger L. King 




UNITED STATES DEPARTMENT OF THE INTERIOR 
Cecil D. Andrus, Secretary 

BUREAU OF MINES 

Lindsay D. Norman, Director 




'\ 



h 









This publication has been cataloged as follows: 



Cooley, Wils L 

Guide to substation grounding and bonding for mine power 
systems. 

(Information circular - U.S. Bureau of Mines ; 8835) 
Bibliography: p. 27- 

1- Electricity in mining- 2. Electric substations. 3» Electric 
currents— Grounding. I. King, Roger L», joint author. II. Title. IIL 
Series. United States. Bureau of Mines. Information circular ; 8835» 



r\2y r ..i:l [TN343I 622s [622'. 48] 80-607913 






^o 



X 



CONTENTS 



surge voltage V is limited to V A for location A, but rises 



to Va + Vg g for location B 

6. Failure of secondary arrester. Fault current If flowing through 



the ground beds can cause potentials V Gl and V G s. V Gl may be as 



Page 



Abstract 1 

Introduction 1 

The substation ground mat. 2 

Grid design 2 

Mat topography 4 

Additional protection 5 

Surge arresters 6 

Sizing of surge arresters 6 

Physical location of arresters 7 

Protection at terminators 7 

Secondary arresters 7 

Grounding distribution system arresters 9 

Safety ground bed 10 

Separation from substation 11 

Measurement of ground bed resistance 11 

Fall-of-potential procedure 12 

Measurement of ground bed coupling 13 

Wiring of ground systems 15 

Wiring the neutral grounding transformer 15 

Insulation of safety ground 16 

Ground-to-ground shock hazard 18 

Ground fault protection 18 

Ground fault relay settings 21 

Multiple grounds 22 

Portable substations and power centers 22 

Grounding the borehole casing 22 

Ground-check monitoring 24 

Monitor connections 24 

Lightning and transient damage 25 

Ground bed corrosion 25 

Bibliography 27 

ILLUSTRATIONS 

1. Typical ground mat for a larger mine substation (24 by 65 feet) 

showing increased protection by adjustment of grid spacing and 

burial depth 5 

2. Typical ground mat for a small mine substation (18 by 30 feet) 

with auxiliary driven rods to lower total resistance 6 

3. Poor utilization of arrester for transformer protection. Lead 

lengths L x and La add about 1.6 kv per foot to transient 

voltage V; L3 and L4 also add some voltage 8 

4. Good utilization of arrester for transformer protection 8 

5. Location of secondary arrester for equipment protection. The 



^r> 



high as 250 v and becomes a hazard throughout the entire mine, ... 10 



ii 



ILLUSTRATIONS --Continued 



Page 



7. Suggested location of safety ground bed with respect to the 

substation 12 

8. Fall-of-potential resistance curves 14 

9. Technique for determination of coupling between ground beds 14 

10. Direct measurement of coupling 15 

11. Incorrect location of derived neutral at a point remote from the 

source transformer 16 

12. Ground-to-ground faults may develop at several places within the 

substation unless the safety ground is well insulated 18 

13. Placement of ground fault CT. It must not be placed at C or D, A 

and B are satisfactory, but A is preferred. The CT must not 

encircle pilot or ground wires. The zero-sequence CT is shown 

at E ,. 19 

14. PT connection for detection of ground fault. Method A is preferred 

since it monitors continuity from the cable ground at the pothead 

all the way to the neutral point on the transformer 20 

15. Use of transformer-coupled grounding resistor for high-voltage 

mine distribution 21 

16. Sketch of parts of the safety ground system, showing various types 

of conductors. Type 1 is crucial and vulnerable. All parts 

that carry ground fault current If should be monitored 24 



GUIDE TO SUBSTATION GROUNDING AND BONDING 
FOR MINE POWER SYSTEMS 

by 

Wils L. Cooley 1 and Roger Lt King 2 



ABSTRACT 

Although electric utility companies have been active in grounding and 
bonding within substations, the mining engineer or mine electrical engineer 
is not involved to the extent that he can be fully up-to-date on the most 
effective practices for substation construction. The coal mine power system 
is grounded in a fundamentally different way from most other industrial power 
systems and is subject to considerable Federal and State regulation. At this 
time, there is little information that is directly applicable to the mine 
situation, especially if the substation must be built in an area of limited 
size or in low-conductivity earth. The objective of this guide is to provide 
specific engineering information to the mining industry. Using as little 
theory as possible, it was written to be general enough to cover most sub- 
stations, but specific enough to provide direct help with each substation. 
It will attempt to recommend practice that is in agreement with present 
Federal rules and regulations. 

INTRODUCTION 

Federal law requires that each coal mine establish a high-resistance 
grounded mine power system at its surface substation. This system is required 
to be connected to a low-resistance grounding medium at the power source. Low 
resistance is usually defined as being 5 ohms or less. Additionally, all con- 
ductors extending underground must be protected with surge or lightning 
arresters, which are to be connected to a low-resistance grounding medium on 
the surface; this grounding medium shall be separated from the power system 
neutral grounds by a distance of not less than 25 feet. (Larger separation 
distances may be used if hazardous conditions can thereby be reduced.) While 
such a requirement has a sound engineering basis for the mining industry, it 
does significantly complicate substation construction. 



^-Professor of electrical engineering, West Virginia University, 

Morgantown, W. Va. 
2 Supervisory electrical engineer, Pittsburgh Research Center, Bureau of Mines, 

Pittsburgh, Pa. 



The two grounds, when required, are meant to serve very different pur- 
poses, and therefore should be designed according to completely different 
standards. The safety ground bed resistance (neutral ground) should provide 
a zero-voltage reference under all conditions, whereas the substation ground 
must sink lightning and fault currents while maintaining the entire substa- 
tion at the same potential. Each of these will be taken up in turn. 

THE SUBSTATION GROUND MAT 

The substation ground mat is a series of interconnected ground wires (and 
possibly ground rods) buried in the earth of the substation floor. To it are 
connected the frames of the power transformer and all substation switchgear, 
surge arresters within the substation, the steel substation structure, the 
substation fence, and static wires of associated pole lines (usually). 

The primary purpose of the substation ground is to provide *electric shock 
protection for personnel working in and around the substation, including 
during lightning strokes, short circuits, equipment failures, and many situa- 
tions of human error or carelessness. This protection is provided by the 
multiple interconnection (grounding) of all accessible surfaces within the 
substation in such a way as to limit to a safe level the voltage differences 
that can appear from point to point on these surfaces. Substation equipment 
that cannot be grounded (the power bus, for instance) is placed so as to be 
inaccessible to contact by personnel. 

A second purpose of the substation ground is to limit insulation stress 
by readily conducting lightning surges to earth via the station surge (light- 
ning) arresters. Although this may be done somewhat more effectively by con- 
structing a very low resistance ground, the improvement is usually not worth 
the effort. On the other hand, if Federal or State regulations require that 
a specified minimum resistance be met, this becomes an additional design 
criterion. 

Grid Design 

One of the most important aspects of substation ground design is the con- 
struction of the ground mat itself, that system of conductors that is to be 
buried within the earth of the substation. The purpose of these conductors is 
to hold the voltage reference of the substation floor at ground potential. A 
common hazard within the substation arises when a person stands on the substa- 
tion floor and touches the metallic surface of the grounded equipment. If a 
large voltage difference exists between hand and foot, an injurious or lethal 
current may flow. These touch potentials can be minimized by covering the 
entire substation with a metal plate that is connected to the accessible 
surfaces, but this is neither practical nor necessary, since the body can 
tolerate some voltage differences without serious effects. An acceptable level 
of personnel protection can be achieved with an open grid of conductors buried 
beneath the surface. The maximum acceptable spacing depends on the available 
fault current, resistivity of the earth, clearing time of protective devices, 



and overall station geometry. All these factors are covered in detail in IEEE 
Standard 80-1976 ( 2) . 3 The follow: 
in nearly every situation, however: 



3 

Standard 80-1976 ( 2) . The following basic guidelines should provide safety 



1. Completely enclose the substation with a continuous ground wire. To 
protect persons outside the station while they are in contact with the station 
fence (assuming the fence is metallic) , this perimeter wire should be buried 
about 3 feet outside the station fence. 

2. Lay out a rectangular grid of wires covering the substation area. 
These should be spaced approximately 5 to 15 feet apart, (IEEE Standard 80- 
1976 gives a method for more precise determination of spacing.) The wires 
should be reasonably uniformly spaced, and preferably located along rows of 
equipment to facilitate the making of ground connections. 

3. Add extra conductors to the grid at the corners of the substation and 
in work areas of the substation, since extra protection is needed there. 

4. Bond the grid together at all intersecting points, using either 
exothermic-welded connections or heavy clamps designed for grounding 
applications. 

5. Bond all equipment within the substation to the ground grid at two 
different points, preferably at points of interconnection of the ground 
grid. Bond the station fence and posts to the ground grid. Bond the station 
fence gates. 

6. If a borehole casing is located within the substation, bond it to 
the station ground grid. 

7. Tie water pipes, gas pipes, and other buried conductors to the station 
ground grid, preferably at several points. (If possible, the substation site 
should be chosen to avoid gas and water pipes.) 

8. Connect rails, telephone cable grounds, and other such conductors to 
the ground grid. Since they may carry hazardous station potentials outside 
the station area, it may be necessary to provide insulated joints or other 
means of isolation as they leave the substation. (Railroad track may require 
very special treatment in order to ensure safety.) 

9. If the station is quite small, or if the soil is subject to severe 
freezing or drying or has very high resistivity, ground rods may be driven 
at mesh points and interconnected with the ground grid. These are most 
effective if driven near the periphery, especially near the corners. Driving 
rods closer together than 10 feet is not effective. 



3 Underlined numbers in parentheses refer to items in the list of references 
at the end of this report. 



10. Be sure that all buried conductors are resistant to corrosion. Never 
mix copper with steel or aluminum. If steel clamps must be used with copper 
conductors, for instance, they will be rapidly destroyed by corrosion unless 
they are protected by an asphaltic coating or cadmium plated. 

Under most conditions grids made with small-diameter wire are sufficient 
to carry current safely. Small wires are easily broken, however. Experience 
has shown that 2/0 wire or larger is usually required for mechanical strength, 
and many companies have set 4/0 as their standard. Prefabricated mesh is 
available that uses numerous smaller conductors welded together. This is also 
acceptable mechanically. The conductors themselves can be copper, copper-clad 
steel, galvanized steel, stainless steel, or aluminum (usually not a good idea 
unless special alloy is used). It must be remembered that dissimilar metals 
must not be buried together as part of the grounding grid. 

The conductors should be buried about 18 to 24 inches below grade in the 
substation. The perimeter cable may be placed somewhat deeper to reduce the 
gradients present around the station perimeter. Close-mesh mats near 
operating handles should be brought up closer to the surface for maximum 
protection. 

Rubber mats or wooden platforms can be used for personnel protection at 
operating handles instead of close-mesh mats, but they must not remain in the 
substation, since they do not protect when wet and are hazards in the winter. 
Care must be taken that the insulated area is large enough to protect the area 
around the handle. 

The use of gravel surface in the substation greatly improves safety, 
since it significantly increases the contact resistance between earth and a 
person's feet. The use of gravel can increase the tolerable touch voltage 
by a factor of two or more. This gravel should be well drained, at least 
4 inches thick, and extend more than 4 feet beyond the substation fence. 
Providing a thicker layer of gravel will afford even more protection. 

Mat Topography 

The total resistance of a ground mat is closely related to its perimeter. 
A long, narrow mat will have lower resistance than a square one of the same 
area. Decreasing the grid spacing will not significantly reduce total resist- 
ance, nor will driving ground rods within the station itself. The mat should 
be designed to cover a relatively large area with conductors arranged to 
reduce high-voltage gradients. Figure 1 and figure 2 show suggested ground 
mat configurations for large and small substations. 

Although the equipotential characteristic of the substation ground mat 
will protect personnel regardless of its resistance to earth, it is desirable 
that the resistance of the substation ground be low. This reduces the voltage 
rise that lightning current causes, which reduces the stress on insulation 
throughout the power system and tends to reduce gradients in the earth around 
and within the substation. For moderate to large substations, the size of 
the grid will usually insure that the station resistance is sufficiently low. 



'Fence 



tv 



/rence 
—ii • ( • • • 1 




HE' 



Borehole 
'casing 



zi 



• — <> 



Main work 
area 



^Operating 



handle 



Large mine substation-Plan view 




Exit area 

(wide enough 
to contain 
open gates) 



Scale, ft 



S 



Fence 



k£k 



Open gate 




Grade level 



Large mine substation -Elevation 

FIGURE 1. - Typical ground mat for a larger mine substation (24 by 65 feet) showing increased 
protection by adjustment of grid spacing and burial depth. 

For small substations, or substations built on high-resistivity soil, it may 
be desirable to augment the earth connection with driven rods or other addi- 
tional conductors. It is recommended that the substation ground resistance 
be reduced to 5 ohms or less where practical. 

Additional Protection 



As mentioned above, additional protection can be achieved by adjusting 
the burial depth and mesh spacing in critical areas. Close spacing and shal- 
low burial are suggested in personnel areas at operating handles, while deeper 
burial is suggested at the station periphery to reduce the gradients along 
the station boundaries. 

Besides being subjected to touch potentials within a protected area, a 
person may experience an excessive voltage between his feet as he enters or 
leaves the protected area. These step potentials can be lessened by pro- 
viding "potential ramps" of progressively deeper buried conductor in the 
exit area. 




Fence 

borehole 

casing 



"^ 



zy 



Exit area 
(wide enough 
to contain 
open gates) 



Small mine substation- Plan view 



1 




Open 
gate 



fc 



Grade level 

• 



Small mine substation- Elevation 

FIGURE 2. - Typical ground mat for a small mine sub- 
station (18 by 30 feet) with auxiliary driven 
rods to lower total resistance. 



Such additional protec- 
tion is necessary only where 
large fault currents 
(greater than 10,000 amperes) 
are possible. 

SURGE ARRESTERS 

Surge arresters are 
important components of any 
substation, and their proper 
interconnection to the ground 
grid is crucial to providing 
the proper level of protec- 
tion for equipment and 
personnel. The term "light- 
ning arrester" does not 
properly describe the func- 
tion of the modern protec- 
tion devices, and they are 
now more properly called 
surge arresters or surge 
diverters, since they are 
designed to divert any 
abnormal surge to ground. 



The modern surge 
diverter is a fairly sophis- 
ticated device which automatically diverts abnormal transients to ground, 
interrupts any power-follow current, and often provides a visual indication 
of its failure when this occurs. Detailed accounts of device operation are 
given in several places and will not be covered here (_8) . 

Sizing of Surge Arresters 

To provide the proper level of protection, arresters must be correctly 
sized for the power system voltage and configuration used. The sparkover 
voltage should be low in order to divert as many surges as possible, but 
must be high enough to prevent operation during normal power system voltage 
fluctuations. In conventional applications direct-connected arresters are 
continuously energized at the line-to-ground power frequency voltage. To 
size the arrester, it is necessary to determine the highest line-to-ground 
power frequency voltage to which the arrester would be subjected during a 
ground fault on any of the system phases. This voltage may range from a 
minimum equal to the line-to-ground voltage on a solidly grounded wye system 
to a maximum equal to line-to-line voltage on an ungrounded delta system. 
Resistance-grounded mine power systems are essentially equivalent to 
ungrounded systems and therefore require that arresters be sized according to 
line-to-line voltage. Sizing of arresters for the primary line (from the 
utility company) will depend on how effectively their system is grounded, 
information which should be obtained directly from the company. If arresters 



are not sized properly, they cannot fulfill their function. If they are sized 
too low, they will operate during ground faults and be damaged. If they are 
sized too high, they will not properly protect equipment. 

Physical Location of Arresters 

Surge arresters must be placed as close as possible to the terminals of 
the equipment they are to protect. Within the substation, arresters are pri- 
marily used to protect the power transformers and will not properly do so if 
placed more than a few feet away. Most manufacturers provide arresters on 
the substation equipment or provide mounting brackets for arresters to keep 
them close to the transformer bushings. It is estimated that the self- 
inductance of each foot of line lead to the arrester and the arrester ground 
lead causes a 1,600-volt drop for a typical lightning surge. As shown in 
figures 3 and 4, this means that the voltage transient produced by lightning 
at a piece of protected equipment is significantly affected by arrester 
location and lead placement. The transient voltage of concern is usually that 
which appears between the power conductors and the case or frame of a trans- 
former or motor. To make this as small as possible, the arrester should be 
placed as directly as possible between these two points. Leads should be run 
as straight as possible with no sharp bends. 

The only exception to this rule occurs when the installation also incor- 
porates a fuse at this location. Since the operation of the arrester dis- 
charges a heavy current which may damage the fuse, it is proper to place the 
fuse between the arrester location and the protected equipment, as in figure 4. 
This increases the transient voltage slightly, but protects the fuse. However, 
this installation may necessitate working on an energized line if the arrester 
requires replacement. 

Protection at Terminators (Potheads) 

Federal law requires that surge arresters be placed on ungrounded con- 
ductors that pass underground, and that these arresters be no more than 
100 feet from the point where the conductors enter the mine. In many mines, 
this provides transient protection for the power distribution cables, but 
some open pit mines require additional protection. In these mines the 
secondary distribution circuit (pit-feed) is carried as open pole line until 
it nears the pit, where it is converted to the shielded feeder cable lying on 
the earth. The cable should be protected by placing arresters as near as 
possible to the cable terminator. These arresters should be connected to the 
substation ground (if it is nearby) or to a separate ground established at 
this location. 

Secondary Arresters 

Since arresters are required on all ungrounded conductors leading under- 
ground, most mines incorporate secondary arresters into their power system 
designs. Whereas primary arresters are very often provided or specified by 
the manufacturer of the substation transformer, the mine operator or engineer 
may be forced into designing his own secondary transient protection. 



«BB 





1 


sn 


o 




u 




(0 












L2- 




o 




i— 


3 


■* 




0) 


O 


_J 




b 


-O 


T1 




o 


u 


c 

D 




i/j 


-a 


ro 




D 


a 


_l 




i_ 


(v 






*" 


_J 


> 




O 


-n 


<D 




1 


c 


U) 




j— 


a 


U 




a> 








+- 




O 
> 




if) 


_J 




1— 


i/) 


c 




n 


, c 








<U 




•-*— 


rn 


■ ~~ 




o 


c 


(/) 


• 


c 


o 


c 
n 


rr 


o 






n 


D 
N 


a 

CD 

1 


o 

-*— 


o 

> 


■*— 




o 
o 


03 

E 
n 


D 


c 


M_ 


O 
O 


o 

•4— 

(J 


a. 




n 


a) 


> 


~a 




-♦— 


_M 


a 



Manufacturers of load centers provide protection on their incoming lines, but 
usually more arresters are needed, especially on the surface as the conductors 
enter the mine. The same rules apply as when sizing and installing the primary 
arresters, except for two important differences. First, since the secondary 
circuit is grounded to the safety ground bed, these arresters offer maximum 
protection for equipment when connected to the safety ground and not to the 
substation ground. However, for reasons of personnel safety, this sometimes 
cannot be done since it may cause large voltages to appear on the safety 
ground. The proper choice of an arrester ground is discussed in the next 
section. 

Second, arresters on the distribution system can be subject to sustained 
overvoltage if there is an insulation failure between the primary and secondary 
circuit. Such an overvoltage can cause the arrester to explode as it attempts 
to carry a heavy sustained current, sending debris flying with injurious force. 
Ferroresonance is also a source of secondary arrester failure. If the arrester 
cannot be located so that its explosion will not injure personnel, then it 
should be protected by a fusible "lockout" connection or other means of 
reducing the probability of secondary arrester explosion. 

GROUNDING DISTRIBUTION SYSTEM ARRESTERS 

Surge arresters used to protect the primary circuit in the mine substation 
should be tied directly to the substation ground grid. When arresters are 
placed on the secondary distribution system, however, care must be taken to 
insure that they provide proper protection without endangering mine personnel 
by causing high voltage to appear on the safety ground (fig. 5). On the other 
hand, one must also be sure that circuit protection will operate when an 
arrester fails, lest this failure cause a lethal shock (fig. 6). 

There are two opposing requirements for grounding of arresters placed on 

the distribution system: 



Phase conductor 




m 

Safety 
ground bed 

FIGURE 5. - Location of secondary arrester for equipment 
protection. The surge voltage V is limited 
to VA^or location A, but rises to Va + V GG 
for location B. 



1. To protect equip- 
ment effectively and provide 
for circuit protection in 
the event of an arrester 
failure, one should ground 
the arrester to the neutral 
reference point (the safety 
ground bed) . 

2. To protect person- 
nel working with safety- 
grounded equipment from 
receiving a shock, one should 
ground the arrester separately 
from the safety ground system, 
thus preventing a current 
flow (and corresponding volt- 
age rise) through the safety 
ground bed. 



10 




x 

o 



'61 



w-zb-V 



=F V 62 



FIGURE 6. - Failure of secondary arrester. Fault current l F 
flowing through the ground beds can cause potentials Vq 1 
and V G2 t V G1 may be as high as 250 v and becomes a 
hazard throughout the entire mine. 



Current practice for ground- 
ing takes three distinct 
forms: 

1. On pieces of portable 
equipment that contain 
arresters (such as power cen- 
ters or draglines) , arrester 
ground connections are made 
to the frame ground (the 
safety ground system) . A 
principal reason for this is 
that no other ground is 
available. 



2. When the secondary 
arresters are placed at pot- 
heads or equipment at the substation, the arresters must be grounded to the 
substation ground mat/ This prevents surges from the source from being com- 
municated to the safety ground system. Since the arresters are connected to 
the station ground, care must be taken to insure that they are not also con- 
nected to the safety ground, since that would destroy the separation required. 
Connection of the arresters to the station ground means that the current-flow 
path in the event of an arrester failure is from one ground bed to the other, 
possibly causing hazardous potentials on both. This condition should not 
persist, and the system must be tested to see that ground fault protection or 
arrester lockout will operate if an arrester becomes shorted. 

3. When arresters are placed on permanent equipment remote from the sub- 
station, they should be grounded to a low-resistance grounding medium separate 
from the safety ground whenever practical. This requires the construction of 
a local ground bed. This bed should be of sufficiently low resistance that 
ground fault protection will operate in the event an arrester fails, shorting 
it to ground. It may be interconnected with the substation ground via a 
static wire or some other means, but it must not be connected to the safety 
ground system; this is not permitted because it allows lightning entry to the 
safety ground. 



SAFETY GROUND BED 

As mentioned previously, the neutral of the mine power distribution system 
must be separately grounded from the substation ground grid. The reason for 
this separation is to prevent any large potentials that may develop on the sub- 
station ground as a result of primary faults or lightning activity from being 
coupled to the frames of mining equipment. Open pit mines may experience 
lightning strokes directly to the secondary system or to equipment frames, 
however, in which case separation affords no extra protection. Depending on 
conditions, therefore, establishing a separate safety ground (neutral ground) 
may or may not provide maximum safety. A separate safety ground appears to be 
generally safer except in situations where the mining equipment itself is very 
susceptible to lightning. 



11 



If the mine power system is to utilize a separate safety ground bed, this 
must be designed according to significantly different criteria than those that 
apply to the design of the substation grid. Its primary purpose is not to 
dissipate lightning or maintain low potential gradients within some area, but 
rather to provide a reference voltage for all the underground mining equipment. 
This voltage must be maintained as close to zero as possible so that no sig- 
nificant potential will exist between the equipment and the mine floor. This 
low potential is to be preserved in spite of the flow of stray and induced 
current through the ground bed. 

The principal design requirement for the safety ground bed is, therefore, 
low resistance to earth. Since current flow through the bed is relatively 
small, potential gradients are of no concern. In fact, the achievement of 
minimum resistance for a given amount of electrode material tends to maximize 
gradients in the vicinity of the bed. The design of an acceptable safety 
ground bed is covered in detail in Bureau of Mines Information Circular 8767 
and will not be repeated in any detail here (6) . 

Separation From Substation 

Federal law requires that the safety ground bed be separated from surge 
arrester grounds (the substation ground) by 25 feet or more. Separation is 
required to reduce the likelihood that a heavy lightning discharge current 
at the substation ground will find its way to the safety ground bed and elevate 
it to dangerous voltage levels. Twenty-five feet has been chosen as the 
minimum separation necessary to assure that dangerous substation potentials 
will not be transferred to the safety ground system. This has been based on 
normal conditions and may not be sufficient for all situations. Mines that 
have a large substation ground and a large safety ground are very likely to 
need longer spacing. It is a good idea to separate the beds by a distance 
equal to twice the maximum dimension of the safety ground bed. The maximum 
dimension is defined as the longest straight line that can be drawn within 
the volume enclosed by the bed. (If the bed is a borehole, the maximum dimen- 
sion is equal to its length.) The separation distance should be measured 
between the two closest points on the respective beds, and the safety ground 
bed should preferably be placed off a corner or narrow side of the substation. 
This ensures that coupling between the two grounds is minimized. Figure 7 
shows this in more detail. 

Measurement of Ground Bed Resistance 

Some regulating agencies require that safety ground bed resistance have 
a resistance less than some standard value, usually 4 or 5 ohms. Mine 
operators are required to measure the resistance of the bed periodically and 
are subject to verification by an inspector. The correct measurement of earth 
contact resistance is not a simple matter, however. It requires special equip- 
ment and specific procedures. 

The major difficulty with making ground bed resistance measurements is 
that the bed has a single terminal and therefore provides no convenient way to 
measure the resistance. Measurement requires a rather complicated arrangement 



12 



i>- 

i it 1 

> ii 1 

ii 




Safety ground 
bed 



/=25 feet or 2d, 
whichever is larger 



Substation ground mat 

FIGURE 7. - Suggested location of safety ground bed 
with respect to the substation. 



of electrodes, with the 
possibility of multiple 
measurements and interme- 
diate calculations. The 
technique is called "fall- 
of-potential. " Although 
several other measurement 
techniques can be found in 
the literature, they are 
subject to severe errors 
and are not recommended. It 
is important that the same 
procedure and electrode loca- 
tions be used in followup 
tests as were used initially. 

The fall-of-potential 
(FOP) technique is complicated 
in that it requires several 
measurements. These multiple 
measurements are its 
strength, however, since they 
provide redundancy in the 
data. This redundancy 
allows the operator to check 
the data against itself for 
consistency. Unrecognized 
factors can cause huge 
experimental errors in 
assessing resistance by any 
technique. The internal 



checking feature of the FOP will detect these errors, 



Use of the FOP technique usually requires a meter designed especially 
for ground resistance measurements. A voltmeter and ammeter may be used in 
rare situations, but this is not recommended. The technique also requires 
a nonconducting measuring tape, two metal stakes, and several hundred feet 
of wire with clamps. 

Fall-of-Potential Procedure 

The following steps are necessary if the measurement is to be accurate. 



1. Disconnect the ground bed from the ground system . Measurements made 
on a connected system do not yield correct results. They always show the bed 
resistance to be lower than it really is because of the connected equipment; 
this may allow a dangerously high bed resistance to go undetected. Extreme 
care should be taken when disconnecting the bed, since the occurrence of a 
fault during this time may produce dangerous potentials on the ground system. 
The mine should be shut down for the period of measurement, or some other 



13 



low-resistance ground bed should be used temporarily. High-voltage gloves 
should be worn when working with ground connections on active systems. 

2. Be certain that the meter is tightly connected to the ground bed by 
short leads . If the meter provides the option of making separate current and 
voltage connections to the bed (a four-terminal meter) , make these connections 
independently rather than using a jumper at the meter. 

3. Estimate the dimension of the ground bed under consideration. As 
before, this is defined as the longest straight-line measurement within the 
volume of the bed. It is the length of any single rod, the diagonal of a 
rectangular grid, the diameter of a circular bed, etc. 

4. Establish a straight line extending from the estimated center of the 
bed for a distance equal to five times the dimension of the bed, or 50 feet, 
whichever is larger. This line should lie on fairly uniform earth wherever 
possible and stay far away from any buried metallic objects such as water 
pipes or other ground beds. Drive one of the stakes a few inches into the 
earth at this point to form the auxiliary current electrode. 

5. Mark off points along the line at 20 percent, 40 percent, 50 percent, 
60 percent, 70 percent, and 80 percent of the distance from the center of the 
ground bed to the auxiliary electrode. 

6. Make a series of resistance readings by driving the voltage electrode 
in at each of the marked points and taking a reading. 

7. Plot the data , graphing resistance versus distance. This should form 
an S-shaped curve with a relatively flat portion at its center. If it does, 
then the true resistance of the ground will be found at the point the curve 
crosses a line 62 percent of the distance to the auxiliary current electrode. 
If the resistance value at 60 percent differs from the value at 50 percent by 
more than 10 percent, or if the curve is not relatively smooth, it suggests 
that the measurement is not satisfactory and should be done again along a new 
line. 

Figure 8 indicates some typical data curves and the conditions that 
produce them. Conditions shown in B and D can usually be remedied by repeat- 
ing the measurement with a longer or different base line. Condition C may 
require a completely different measurement procedure, which is beyond the 
scope of this discussion. 

Measurement of Ground Bed Coupling 

As mentioned previously, insufficient spacing between the safety ground 
bed and the substation ground mat may allow dangerous potentials to appear on 
the safety ground as a result of current flow from the substation ground. If 
one suspects that too much coupling exists between the grounds, it can be 
measured either directly or indirectly. 



14 



LlI 

o 



c/5 

UJ 
IT 

Q 
UJ 
OC 

3 

< 

Ul 




20 40 60 80 

DISTANCE FROM GROUND BED TO 
AUXILIARY CURRENT ELECTRODE, pet 

FIGURE 8. - Fall-of-potential resistance curves. 



To measure Ri 



100 



77777 

Substation 
ground bed 




Safety 
ground bed 

FIGURE 9 



Technique for determination of coupling 
between ground beds. 



For the case where the 
safety ground bed is isolated 
from the ground system, it 
can be shown that the poten- 
tial of the safety ground 
bed will rise to a value V, 



V = V r 



where V r 



R, + R 2 - R 



11 



R 



12 



Rn 



and R 2 



2Ri 

voltage on the 
substation 
ground mat, 

measured resist- 
ance between 
safety and sub- 
station 
grounds , 

measured resist- 
ance of the 
substation 
ground , 

measured resist- 
ance of the 
safety ground. 



(1) 



R-l and R 2 can be meas- 
ured by the techniques 
described above, and R l3 is 
easily measured also (fig. 9). 
Since the safety ground bed 
will not be isolated when in 
use, equation 1 overestimates 
the coupling but should be 
able to verify the approxi- 
mate value. 

A direct measure of 
coupling can be obtained by 
circulating a large test 
current between the substa- 
tion ground and some auxil- 
iary current electrode far 
from the substation. The 
resultant voltage can then 
be measured between the sub- 
station and a remote voltage 
probe and between the safety 



15 



Remote voltage 
probe 



Safety 

ground 

bed 




Substation 
ground bed 



Source of 
current 



bed and the remote probe, as 
shown in figure 10. This 
should directly indicate the 
coupling. No criteria have 
ever been set for maximum 
acceptable coupling, but 
mines with values as high as 
15 to 20 percent have 
apparently not experienced 
any disastrous effects. 

WIRING OF GROUND SYSTEMS 

In addition to consid- 
erations of ground bed size, 
location, and construction, 
the design of a mine substa- 
tion requires attention to 
other details of implement- 
ing the ground system. These 
are covered in a series of 
short paragraphs. 

Wiring the Neutral 
Grounding Transformer 



Remote 
current electrode 

FIGURE 10. - Direct measurement of coupling. 



Regulations require a 
resistance-grounded power 
system for almost all mining 
operations. The only excep- 
tions pertain to stationary 
equipment. This resistance-grounded system may be implemented either by Y- 
connection of the three-phase source transformer secondary or by the use of 
a separate trans former (s) to derive a neutral for the mine power system. If 
the neutral is derived separately from the source transformer, then certain 
precautions should be taken to insure the safety of the power system. Although 
Federal regulations state that the neutral shall be derived at the source 
transformer, some operators have chosen to place the neutral grounding trans- 
former in some other location, such as depicted in figure 11. Although this 
technique allows for a large separation between the safety ground and substa- 
tion ground, it can result in a shock hazard under some conditions. 



The most serious hazard occurs if one or more leads of the grounding 
transformer become disconnected from their phase conductors. The grounding 
transformer then loses its ability to derive the neutral point. A current 
results that flows in the safety ground system, and hazardous potentials 
may appear on the frames of mining equipment. This particular type of failure 
is not restricted to remotely located grounding transformers and must be con- 
sidered even when the grounding transformer is located close to the source 
transformer. The grounding transformer must therefore be of sufficient 
capacity to indefinitely sustain current flow from ground faults. It shall 



16 



Static wire 



Source 
transformer 




//////// 

Substation 
ground bed 



FIGURE 11. 



rm 

Safety 
ground bed 

Incorrect location of derived neutral at a point remote from the source 
transformer. 



not be protected by fusing its legs but should be protected by overcurrent 
relays to trip the main current breaker if excessive current flows in one or 
more of its legs (a result of an open on some other leg). It is necessary to 
take great care when using a separate grounding transformer. 

Hazards can also occur if a line were to fail between the source trans- 
former and the point where the grounding transformer is connected. The 
probability that this will occur is directly related to the length of pole 
line between source transformer and grounding transformer. Besides destroying 
the ability to derive the neutral point, an open phase conductor will disrupt 
power to the mine. If the mine power system is provided with single-phase 
protection or ground fault protection (if the line contacts earth) , the main 
breaker may trip. If not, a hazardous condition can exist for an extended 
period. It is therefore recommended that the grounding transformer be located 
as close as possible to the source transformer. 

Insulation of Safety Ground 

As stated before, the purpose of establishing a separate safety ground 
bed is to prevent hazardous potentials existing on the substation ground (as 
a result of a primary line fault or lightning activity) from being coupled 
to the frames of mining equipment. It is crucial, therefore, to insure that 
no inadvertent contact is made between the substation ground and the safety 
ground. 



17 



Failure to properly insulate the safety ground is a common occurrence and 
may be extremely hazardous. The safety ground wire must never be used as a 
static conductor, or connected to the static conductor, since this deliberately 
exposes the safety ground to lightning. When running a safety-grounded circuit 
on poles, the safety ground wire must be insulated and protected, preferably 
below the phase conductors. 

A hidden source of improper connection between the safety ground and the 
substation ground can occur if belts are used for haulage of coal out of the 
mine to the preparation plant. Very often the conveyor frame is tied to the 
building ground or otherwise tied into the substation ground. If the belt 
drive motors are powered from the safety-grounded mine power system, their 
frames will be connected to the safety ground. Since the motors are normally 
mounted directly to the conveyor frame, a ground interconnection occurs, 
defeating the purposeful isolation of the safety ground. 

It should be obvious then that no direct connection should be made between 
the two grounds and cause a ground-to-ground fault. This fault would couple 
the grounds together at the very moment when isolation becomes critical and 
can completely negate all efforts to provide protection to mine personnel. 

It is extremely difficult to insulate the safety ground system to a 
voltage level that would prevent a lightning-produced arc from occurring. 
Lightning transients are extremely short, however, and do not propagate very 
far along static lines. Primary faults can produce high potentials on the 
substation ground for much longer times, especially if conditions are such 
that circuit protection does not operate. Depending on conditions, such a 
primary fault within the substation can cause the potential of the ground 
grid to approach primary line-to-neutral voltage. Thus it is proper to 
insulate the safety ground to the same level as the primary circuit within 
the substation. This level of insulation protection may be reduced if it can 
be demonstrated that such high voltage levels cannot appear on the ground grid. 
Figure 12 shows several areas in the substation where ground-to-ground faults 
are most likely to occur. 

The cable connecting the safety ground bed to the ground system is quite 
often placed near or directly on the bare metal of the substation as it is 
carried out of the substation, which means that the cable insulation itself 
must withstand the voltage difference between grounds (A). Insulation rated 
at 600 to 2000 volts is often used, and this is simply not enough to guarantee 
protection. There is also a possibility of developing a ground-to-ground 
fault between the borehole casing (or Kellems grip) and the shield of the 
borehole cable (B) , and care should be taken to see that these are adequately 
insulated. The neutral grounding resistor is a third source of possible 
ground-to-ground faults, since it may only be insulated for secondary line- 
to-line voltage (C) . 

A fourth source of ground-to-ground faults is the ground-check monitor 
installed in the substation (D) . Most monitors are designed so that their 
cases and power supplies are connected to the substation ground. Since they 
monitor the continuity of the safety ground wire, the internal circuitry is 



18 



/^ 



Safety ground wire 




m 

Safety 
ground bed 



Substation ground bed 



FIGURE 12. - Ground-to-ground faults may develop at several places within the substation 
unless the safefy ground is well insulated. 

is connected to the safety ground. Lightning transients and faults often 
damage the delicate circuitry of the monitors. In an attempt to protect the 
monitors, manufacturers have installed various surge protectors between the 
monitor case and the safety ground. This practice can cause a hazard to 
personnel, however. If a very large transient occurs, it can cause some types 
of surge protectors to fail short. This short permanently connects the two 
grounds together, negating all efforts to provide a safe system. Unfortu- 
nately, failure of a surge protector can go undetected indefinitely. If surge 
protectors are installed between the two grounds, they should either be a type 
that does not fail short or be checked for failure at frequent intervals. 

Ground- to-Ground Shock Hazard 

The previous paragraphs suggest that it may be required in some circum- 
stances to deliberately isolate the two grounds when they appear in close 
proximity. Although isolation of monitor frames, motor frames, etc. , is 
necessary to preserve the properties of the safety ground, a severe shock 
hazard can exist at such locations, since it would ordinarily be possible for 
personnel to simultaneously contact both grounds. Contact should be prevented 
where possible, and clear warning of danger posted. 

Ground Fault Protection 



Ground fault protection is normally carried out using one of three 
approaches : 



19 



1. A window-type current transformer (CT) encircling the three phase 
conductors to detect any zero-sequence current. 

2. A bar-type CT in the grounding resistor lead to detect any neutral 
current. 

3. A potential transformer (PT) across the grounding resistor to detect 
any potential difference between neutral and ground. 

These methods all have equal ability to detect simple ground faults, but 
they differ somewhat in their reliability under adverse conditions. 

The zero-sequence CT and the ground-lead CT are essentially equivalent, 
except that care must be taken to properly place the ground-lead CT. Figure 13 
shows the details of CT placement. Positions C and D are not correct since 
they detect ground current rather than neutral current. Position A is pre- 
ferred over B since it is closer to the transformer neutral and can detect 
neutral current in spite of a short in the grounding resistor circuit. It 
should be obvious that only the phase conductors are to pass through a window- 
type transformer. This means that high-voltage cables must not be shielded 
where they pass through the CT, or care must be taken to see that the shield 
is not terminated so as to produce a shorted turn through the CT. 



Phase conductors 





To mine 



To safety 
ground bed 



FIGURE 13. 



Short J 



m 



Neutral 

grounding 

resistor 



C 



B 



PS\ 



To mine 



D 



Safety 
ground wire 

Placement of ground fault CT. It must not be placed at C or D. A and B are 
satisfactory, but A is preferred. The CT must not encircle pilot or ground 
wires. The zero-sequence CT is shown at E. 



20 



A PT across the grounding resistor is often considered to be the safest 
method of detecting a ground fault. If the ground system becomes open, dan- 
gerous conditions can exist in which no neutral current can flow since the 
loop is open. Neither CT approach will detect such a condition. A potential 
will exist across the open, however, and can be detected by a PT. Figure 14 
shows the preferred PT connection, in which the connections are made to pro- 
vide verification of continuity for a large portion of the ground circuit. 

The PT method of detecting ground faults can fail also if the leads to 
the PT become open, so they should be relatively well protected. The highest 
level of protection can be achieved by using both CT and PT protection. This 
redundancy makes the ground-fault detection system highly reliable. 

Neutral grounding resistors must be sized to carry maximum ground fault 
current for an extended period. If ground fault protection does not operate 
for some reason, the resistor must also have an adequate voltage rating for 
the system (line-to-line). On very high voltage resistance-grounded systems 
it may be necessary to use a lower voltage resistor that is transformer- 
coupled to the neutral point of the source transformer (fig. 15). In this 
case the resistor must carry current equal to the maximum ground fault times 
the turns ratio of the transformer and should be rated accordingly: 

KVA rating = I 6f x V L N • 




Pt 



Neutral 
grounding ^p+ 
resistor > 



B 



Safety 
ground wire 



To safety ground bed 



Borehole 
or cable 




FIGURE 14. - PT connection for detection of ground fault. Method A is preferred since 
it monitors continuity from the cable ground at the pothead al I the way to 
the neutral point on the transformer. 



21 



Secondary 




Neutral 

grounding 

resistor 



The coupling trans- 
former should also be rated 
for full continuous current 
and insulated for line-to- 
line voltage. PT ground 
fault protection should 
ideally go across the trans- 
former primary, and CT 
protection is best placed 
in a series with the 
primary. If the CT is 
placed on the secondary, 
it should be sized 
accordingly. 



Ground Fault Relay Settings 



Safety 
ground bed 

FIGURE 15. - Use of transformer-coupled grounding 
resistor for high-voltage mine distribution. 



Single- phase 
ground coupling 
transformer 

Proper adjustment of 

ground fault protective 
equipment is often diffi- 
cult because of the low 
level of fault current. 
Many of the techniques that work well for more solidly grounded systems work 
either poorly or not at all when used with high-resistance grounding. 
Neutral grounding resistors are usually chosen to limit the fault current 
from 15 to 25 amperes. If a 25/5 or 50/5 current transformer is used, this 
results in a relay current of as little as 1.5 amperes. It is customary to 
set the relay pickup at 50 percent of the maximum current, which would be 
less than 1 ampere. This is a very small current for relay operation and 
requires a high-burden relay. High-burden relays often cause the current 
transformer to saturate, causing a loss of calibration. A workable system 
can be implemented using a low- ratio bar-type current transformer, but the 
use of a high-ratio window CT with an improperly chosen relay may cause 
serious problems. Residual flux methods usually do not have the proper 
sensitivity for mine application. 



If the mine power system employs several levels of ground fault protec- 
tion that must be coordinated, the choice of current- transformer ratios and 
relays becomes extremely difficult. 



22 



Multiple Grounds 

The configuration of some mine power systems requires that multiple grounds 
be established. Very often the additional grounds are lightning arrester 
grounds at potheads and service entrances. These additional grounds may or 
may not be interconnected with the substation ground via a static wire or 
other means. If they are part of the substation ground, they may develop a 
hazardous potential during a primary fault. They also extend the size of the 
substation ground, making it very difficult to measure its resistance properly. 
This also increases the coupling between the substation ground and safety 
ground bed, possibly to a dangerous level. 

If these additional arrester grounds are not tied to the substation 
ground, then they must each have a low value of resistance. As discussed 
previously, these beds must be capable of passing enough current in the event 
of an arrester failure to operate the ground fault protection. If the ground 
is not for an arrester, but is simply to provide personnel protection at an 
operating handle, then it need not meet any specific resistance requirements. 

The safety ground system is almost always multiple-grounded because it 
is connected to equipment frames in contact with the earth, but multiple 
surface safety ground beds are rare. Besides enlarging the effective size of 
the safety ground bed and thereby increasing coupling to the substation 
ground, the chief disadvantage of a multiply connected safety ground is a 
difficulty in implementing a ground-check monitor program. Some monitors have 
been conceived that can operate in the presence of multiple connections to 
earth, but they have not yet worked satisfactorily in the field. 

Portable Substation and Power Centers 

These pieces of equipment usually have only one ground, which must be 
considered the safety ground whether or not it is used as a ground for surge 
arresters. This is usually not a problem, however, since they are coupled 
to other equipment through cables, which have a high surge impedance, meaning 
that the current that must flow through the ground is small. The chance of 
a portable substation being struck by lightning is very remote, so this is 
not considered a problem. Having a separate ground for surge arresters in 
underground load centers is simply not necessary. 

As with permanent substations and other equipment, care should be taken 
to insure that grounds are multiply bonded to the frame to reduce the possi- 
bility that protection will be lost. 

GROUNDING THE BOREHOLE CASING 

The borehole casing of an underground mine offers a unique grounding 
problem since it can provide a low-resistance path from the substation area 
directly to a working area of the mine. It is difficult to address all 
possible situations, so only a few will be covered. Generally, if the casing 
is accessible on the surface it must be made inaccessible underground, or 
vice versa. Personnel protection cannot be provided at both ends of the same 
casing. 



23 



Many boreholes are located within the main substation. Since the casing 
is located within the substation ground mat, a serious personnel shock hazard 
exists. Unless the casing is bonded to the substation ground (as shown in 
figures 1 and 2) , a person standing within the substation and touching the 
casing may receive a lethal shock. The casing may be made inaccessible by 
placing it in an enclosed vault, but it is still likely to be closely coupled 
to the substation ground and cable-supporting structures and should be bonded 
to them. 

If the borehole casing is bonded to or closely coupled to the substation 
ground mat, it then has the capability of carrying dangerously high potentials 
underground. It should therefore be treated accordingly. It must not be used 
as a ground reference for any underground equipment or haulage system. It 
should be made inaccessible and labeled as dangerous. If a high potential 
were to occur on the casing, the field would extend into the earth all around 
it, so that the area within at least 10 feet of the casing should be avoided. 

If a borehole casing is located outside the substation, it should not 
be connected to the substation ground unless the ground mat is extended to 
surround the casing. Instead, the casing can either be connected to the 
safety ground or left ungrounded. If the borehole is 50 feet or more away 
from the edge of the substation mat, it can probably be made a part of the 
safety ground bed. Since it is usually quite large compared with the safety 
bed, incorporating it into the bed can significantly reduce safety ground 
bed resistance. It must be verified that coupling to the substation ground 
(discussed earlier) is satisfactorily low, however, and will probably require 
at least a 50-foot separation. The borehole cable supporting structure should 
also be bonded to the casing, and great care must be taken to assure that 
this is not also attached to the substation ground and that surface personnel 
cannot simultaneously touch it and the substation ground. Nor should this 
structure be located where it can contact a primary line. The small increase 
in lightning exposure caused by connecting casing and cable support to the 
safety ground is offset by the low resistance provided. If the casing is 
connected to the safety ground, then it can properly be used as a ground 
reference point at the mine bottom. 

If the casing is outside the substation, it can be left ungrounded, that 
is, unattached to either the substation or the safety ground. Depending on 
its location relative to the substation, it can develop a hazardous potential 
during lightning or faults. If the casing is left ungrounded it should be 
avoided at both ends, but not as scrupulously as when it is specifically 
grounded. 

If boreholes are used to carry dc power underground from the surface, 
they deserve careful treatment also. If the borehole additionally carries 
ac power within the mine substation, then it should be grounded according to 
the previous discussion. If the borehole is dc only and/or need not be 
grounded on the surface, it can be placed in parallel with whichever dc lead 
supplies the rail underground. It will thereby reduce the impedance of the 
dc circuit and help maintain the haulage rail at earth potential. This 
practice should improve the performance and safety of the dc system. 



24 



If the borehole is more than 100 feet from the substation, it may be 
necessary to locate a surge arrester at the borehole. This arrester must not 
be grounded to the borehole structure, but must have its own ground bed 
25 feet away. 

GROUND-CHECK MONITORING 

Because the safety ground system is essential to personnel protection, 
it is important that its continuity be preserved. Since the grounds are sub- 
ject to accident and abuse, it is necessary to monitor safety ground continuity 
on a continuous basis. Monitoring is often a difficult endeavor, however, 
and must be carried out very carefully within the surface substation. 

Monitor Connections 

Because continuity is essential to the safety ground system, as much of 
the system as possible must be checked. There are really three classes of 
conductors within the safety ground system, as shown in figure 16. Highest 
monitoring priority goes to those portions of the system that are essential 
for safety and vulnerable to damage or abuse--the ground wires of the trailing 
cables themselves. Other portions of the ground system are just as crucial, 
but are fairly well protected, such as the ground connections within power 
centers and switch houses. In the third group are those conductors that are 
often vulnerable but are somewhat less likely to cause a severe hazard if 
continuity is lost. This includes the lead from the neutral grounding 
resistor to the safety ground bed and the connections of the bed itself. 



Substation 




Safety 
ground bed 



y^- 



Type 
FIGURE 1 



Type 2 



Type 



6. - Sketch of parts of the safety ground system; showing various types of conductors. 
Type 1 is crucial and vulnerable. All parts that carry ground fault current | p 
should be monitored. 



25 



These conductors are usually less important because in most ground fault 
situations they are not part of the current path. If an arrester failure or 
other unusual fault occurs that requires the safety bed to carry current, 
these conductors become important also. 

Two guidelines to follow in making monitor installations are to monitor 
as much of the system as possible and to carefully protect any part that 
cannot be monitored. Often a simple change of design can significantly 
increase monitor coverage, such as separately connecting pilot and ground 
wires to an equipment frame instead of connecting them both to the same stud. 
When locating monitor connections within the substation, no portion of the 
safety ground should be left unmonitored unless it cannot be included without 
affecting safety in some other way, such as reducing monitor sensitivity to 
parallel paths. 

There is not yet any good method to monitor safety ground bed continuity, 
but periodic (every 6 months) checks of resistance should uncover most such 
problems before they have serious consequences. 

Lightning and Transient Damage 

Experience has shown that monitors located in surface substations are 
subject to a high rate of failure due to transient voltages arising from 
lightning activity or system faults. As mentioned briefly earlier, the 
substation monitor case is usually connected to the substation ground, while 
its internal circuitry is connected to the safety ground. Since the whole 
purpose of separated grounds is to prevent high-voltage surges from being 
coupled from one ground to the other, a well-designed system will often 
experience significant voltage differences. Based on field measurement and 
research literature, it was estimated that a typical open pit substation 
experienced two surges per year in excess of 1,000 volts from lightning alone 
(I) , which can seriously damage most monitors. It is strongly suspected that 
monitors have been destroyed by ground-to-ground voltages during system faults , 
also. 

If the monitor cannot be double-insulated to provide personnel protection 
while avoiding contact with the substation ground, then it must be protected 
by surge arresters or suppression devices. As mentioned earlier, these can 
present a serious hazard if they fail short, and must be chosen carefully. 
Since most surge suppression devices can fail short, they should be placed in 
conjunction with series impedances to limit the energy they are subjected to, 
or be designed so that a failure can be detected. 

GROUND BED CORROSION 

Most mines with underground dc haulage systems experience stray current 
flow in the safety ground as the result of leakage from the track. The cur- 
rent flow pathway can include the safety ground bed itself with disastrous 
corrosive effects. Even if no dc is present, the wrong choice of metals can 
cause corrosion of the ground bed. 



26 



The entire ground structure in and around a substation should be con- 
structed of the same material or compatible materials if possible. Very often 
copper is used for beds even when steel is buried in the vicinity. This 
usually works satisfactorily, because when these two metals are connected 
together, the copper will cause the steel to corrode but will not be harmed 
itself. Since the steel is usually quite massive (such as the substation 
structure, fence posts, or borehole casing), the corrosion has little effect. 
Aluminum has approximately the same effect as copper on steel, but it can be 
rapidly corroded by mine acid. If copper conductors are secured with steel 
clamps, serious problems can arise. Since the copper will cause the steel to 
corrode, these clamps will soon be destroyed, destroying the integrity of the 
bed. 

Proper bed construction requires the use of compatible connectors. Com- 
mercial thermit 4 weld connections and most specially designed ground clamps 
are compatible, but one should be certain. If any doubt exists, the connec- 
tions should be protected by applying a watertight coating of asphalt or some 
other compound before burial. If this cannot be done, connections should all 
be made above the surface of the earth, which will reduce corrosion and allow 
for visual inspection. Exposed connections are not necessarily ideal, however, 
since they may be damaged by activity in the substation area. 



4 Reference to specific trade names does not imply endorsement by the Bureau 
of Mines. 



27 



BIBLIOGRAPHY 

1. Cooley, W. L. , M. M. Hassan, and K. Y. Lee. Ground Wire Monitors for 

Surface Coal Mines. Paper in Mine Power Systems Research (in Four 
Parts). 2. Grounding Research, compiled by Staf f--Mining Research. 
BuMines IC 8800, 1979, pp. 25-26. 

2. Institute of Electrical and Electronic Engineers. IEEE Guide for Safety 

in Substation Grounding. IEEE No. 80-1976. New York, 1976, 76 pp. 

3. . IEEE Recommended Guide for Measuring Ground Resistance and 

Potential Gradients in the Earth. IEEE No. 81-1962, New York, 1962, 
19 pp. 

4. . IEEE Recommended Practice for Grounding of Industrial and Com- 
mercial Power Systems. IEEE No. 142-1972, New York, 1972, 95 pp. 

5. James G. Biddle Co. (Plymouth Meeting, Pa.). Booklet 25T, 1970, 48 pp. 

6. King, R. L. , H. W. Hill, Jr., R. R. Bafana, and W. L. Cooley. Guide for 

the Construction of Driven-Rod Ground Beds. BuMines IC 8767, 1978, 
26 pp. 

7. Lordi, A. C. How to Safety Ground Mine-Power Systems. Coal Age, v. 68, 

September 1963, pp. 110-117. 

8. Ohio Brass Co. (Mansfield, Ohio). How Does a Distribution Class Surge 

Arrester Work? 47 pp. 

9. Powell, G. L. The Safety Grounded System. W. Va. Univ. Mining Series, 

1972, 88 pp. 

10. Sunde, E. D. Earth Conduction Effects in Transmission Systems. Dover 

Publications, Inc. , New York, 1968, 370 pp. 

11. Tagg, G. F. Earth Resistances. Pitman Pub. Corp. , London, 1964, 258 pp. 



INT.-BU.OF MINES, PGH.,P A. 24918 



n 



■ 



4> V ° 















c v / 



o 






t j. • . . • * «& ^ ' 



y\. %^?V v^-v V*^V v^V v 








0* 






^ 



\^\ : -SK ; /\ -I5# ^\ : « ; /\ ° : 

/ V^ T V" V'^V V^ r V K V^^V *V*^V 










o' 
^ 

r 










j? \ .y&p.' ■>* ■%. -.sap,' j> \ 
„* .....\,"-'\^.....^^^ o *°.....\ 






«5*, -a. ' . 

1* .>>*, 







S^ JAN 82 

KsJHK N. MANCHESTER, 
^^ INDIANA 46962 













:^ 



